RewriteCond %{HTTPS} !=on
RewriteRule ^.*$ https://%{SERVER_NAME}%{REQUEST_URI} [R,L]
You need to copy (or modify) a .htaccess file to the root of the directory with the same name as the domain name. At the very top of the file should be:
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^.*$ https://www.%{SERVER_NAME}%{REQUEST_URI} [R,L]